ground improvement techniques for ece 2 _isgi09_
TRANSCRIPT
International Symposium on Ground Improvement Technologies and Case Histories (ISGI09)
GROU�D IMPROVEME�T TECH�IQUES FOR EAST COAST EXPRESSWAY 1
PHASE 2, MALAYSIA 2
3
4
5
Y.W. YEE * and C.G.CHUA* 6
*Keller (M) Sdn. Bhd., 7
Kuala Lumpur, Malaysia 8
The East Coast Expressway (Phase 2) is 190km long and when completed, will connect Jabur Interchange to 11
Kuala Terengganu. Ground treatment works were instituted where the highway passes through swampy ground 12
and soft alluvial, in particular where high fill embankments were constructed. The objectives are to ensure 13
embankment stability and restrict settlements to within acceptable limits. Several types of ground improvement 14
techniques were implemented such as vertical drains and surcharging; vibro sand and stone columns and 15
dynamic replacement. Typically, treatment depth ranged from 4 to 16m depth. This paper describes the design 16
and construction of the ground improvement methods including quality control measures and insitu tests. The 17
embankments were instrumented and monitored during construction to ensure performance was according to 18
design requirements. 19
Keywords: embankment; soft ground; ground improvement; prefabricated vertical drain; vibro stone 20
columns; vibro sand columns; dynamic replacement. 21
1.0 Introduction 22
The East Coast Expressway, Phase 2 (ECE 2) transverses 190 km long from Kuantan to 23
Kuala Terengganu. It complements Phase I of East Coast Expressway (ECE 1) which 24
connects Karak to Kuantan. When completed in 2011, it is expected to act as a catalyst to 25
stimulate the economic growth of east coast of Peninsular Malaysia, particularly the state 26
of Terengganu. 27
28
The ECE 2 was designed as a four-lane dual carriageway with an average width of about 29
32m. The finished level was required to be higher than the flood level of 100 year return 30
period as the east coast of Malaysia is often inundated by flood during the annual 31
monsoon season. 32
33
The structures of ECE 2 comprise of bridges, culverts, elevated structures, fill 34
embankments and cut slopes. For the areas where fill embankments crosses swampy 35
ground and soft alluvial sediments, various ground improvement techniques were 36
employed to ensure embankment stability and restrict post-construction settlements to 37
within acceptable limits. 38
The ECE 2 project was demarcated into 12 separate sub-packages under distinct work 1
contracts. This paper presents the application of various ground improvement techniques, 2
namely (i) Prefabricated Vertical Drain, (ii) Vibro Sand Column, (iii) Vibro Stone 3
Column and (iv) Dynamic Replacement in 6 packages (2, 3, 9, 10, 11 &12). The design 4
and construction of each ground improvement technique are described including relevant 5
quality control procedures and insitu tests. The performance of various ground 6
improvement techniques is explained from ground movement monitoring data. 7
Figure 1. Typical cross section for Vibro Stone Column (left) and Dynamic Replacement Column (right). 8
2.0 Fill Embankments Treated Using Ground Improvement Technique 9
Typical cross sections of embankment fill are shown in Figure 1. Generally, each fill 10
embankment has 32m width carriageway at the top and slopes with gradient 1(V):2 (H). 11
3.0 Subsoil Conditions 12
The treatment areas were generally located on swamps and soft alluvials. The geological 13
map shows that the bedrock comprise of sandstone or granitic formation. 14
15
Typically, soft ground comprises silty clay (cu = 10 to 15kPa) down to varying depths 16
(6m to 16m). In general, water content is 50 - 60% and plasticity index 20 - 30%. 17
4.0 Performance Criteria 18
The roadways in ECE 2 are required to comply to performance criteria set by the Public 19
Works Department (JKR) and Malaysian Highway Authorities (MHA). In general, the 20
maximum allowable differential settlement is 100mm over a length of 100m (1 in 1000) 21
along the centerline of embankment; and the overall embankment stability is required to 22
achieve a factor of safety of 1.5. 23
5.0 Application of Ground Improvement Schemes 24
Various ground improvement techniques were used i.e. Prefabricated Vertical Drain 25
(PVD), Vibro Sand Columns (VS), Vibro Stone Columns (VR) and Dynamic 26
Replacement (DR) Columns. The selection of suitable technique was dependant on 27
factors such as embankment height (weight), soil conditions (strength and depth) and 1
availability of material. Cost factor was a prime consideration as well. 2
5.1 Design of Ground Treatment 3
5.1.1 Prefabricated Vertical Drain (PVD) 4
This technique was widely used because of its relatively low cost and quick installation. 5
Application was mainly for low embankments up to 4 to 5m high. The embankments 6
were built slowly using staged construction to allow the underlying soil to consolidate 7
and gain strength. A surcharge of about 1m is normally placed to accelerate consolidation 8
over a period of 3 to 6 months and limit long term settlement. Spacing between PVD 9
points are typically 1 to 1.2m c/c triangular grid. 10
11
In certain locations, embankments up to 8m were constructed with PVD in combination 12
with VR (VR below the embankment slope and PVD across the carriageway). Basically, 13
VR was designed to ensure stability and PVD to accelerate settlements. Such an 14
application achieves greater economy but has a drawback of slower construction time. 15
16
5.1.2 Vibro Sand Columns (VS) 17
This technique was employed mainly where sand supply was freely available to stabilize 18
embankment fill between 5m and 10m high. The VS design was based on Priebe’ (1995) 19
method to work out the improvement factor in terms of settlement estimates and to derive 20
composite parameters for slope stability analysis. The diameter of VS is 0.9m and typical 21
spacing ranged between 1.6m c/c and 2.0m c/c. Surcharge period of 2 to 3 months are 22
typical. 23
24
In certain locations, combinations of VS and VR were used to stabilize embankment fill 25
to maximum height of 13m, where VS was specified across the carriageway to reduce and 26
accelerate settlement and VR was specified at embankment slope to ensure slope stability. 27
28
5.1.3 Vibro Stone Columns (VR) 29
Vibro Stone Columns were designed to support high embankment fill over very soft 30
ground. The design methodology adopted Priebe’s (1995) method. The diameter of VR is 31
1.0m and typical spacing ranged between 1.8m c/c and 2.4m c/c, depending on 32
embankment height. High embankments up to 13m have been designed for this project. 33
Rest period for such high embankments are typically less than 1 month. 34
35
5.1.4 Dynamic Replacement Columns (DR) 36
Dynamic Replacement is basically an extension of the Dynamic Compaction method 37
which is generally applied in granular soils. As soft fine grain soils cannot be compacted, 38
the DR method is an improvisation to use the tamper to install granular columns in the 39
ground. The depth of installation is limited to 6m (as explained in paragraph 6) and 40
hence, is suitable for soft soils of shallow depths only. The columns are irregular shaped, 41
typically 2.5m in diameter and spaced at 5.5m centres. Since the clear spacing between 42
columns is generally 3m apart (compared to 1m for PVD and VR), staged construction or 1
a longer surcharging period of about 4 to 6 months is normally required. 2
3
For soft soils deeper than 6m, consideration was given to design DR in combination with 4
PVD but site trials showed that ground movement was very high in all directions. It is 5
therefore probable that PVDs installed would suffer excessive movement (kinking and 6
creasing) that their functionality would be questionable. Hence, this hybrid solution was 7
not accepted by the engineers involved. 8
Figure 2. “Dry” Vibro Stone Column Rig Figure 3. Dynamic Replacement Rig 9
6.0 Execution 10
6.1 Prefabricated Vertical Drain (PVD) 11
The execution of Prefabricated Vertical Drain was carried out using conventional stitches 12
mounted on light cranes. PVD was designed to be installed to medium stiff / to stiff layer. 13
6.2 Vibro Sand Columns (VS) 14
The execution for Vibro Sand Columns was similar to Vibro Stone Columns as described 15
in literature (BS EN 14731: 2005, Yee & Raju (2007)). The challenges in the execution 16
of Vibro Sand Columns were delivery of sand material to the tip of vibrator and 17
achieving the required compaction effort. Site trials concluded that well-graded dry sand 18
material having fines content not exceeding 10% was required to ensure properly 19
compacted columns. Regulated air-pressure was incorporated during the construction 20
process to avoid any heaving of adjacent vibro sand columns. 21
6.3 Vibro Stone Columns (VR) 22
Vibro Stone Columns (VR) were installed using bottom feed method (BS EN 23
14731:2005, Yee & Raju (2007)). Stones were fed at the top of vibrator and discharged 24
directly to its tip through a special delivery tube attached to the vibrator. This method was 25
pure displacement method where no soil is removed. A “dry” system (without water 26
jetting) was specified by the authorities as an environmental protection requirement. 27
6.4 Dynamic Replacement Columns (DR) 1
Dynamic replacement columns were installed by inserting granular material into the 2
ground from ground surface via repetitive pounding (BRE 458, (2003)). As illustrated in 3
Figure 4, 15 tons to 25 tons tamper was repeatedly lifted between 10 to 15m by a 4
specialised Dynamic Compaction crane and freely dropped onto the ground to form a 5
crater which was then filled up by granular material. This process was repeated until 6
sufficient amount of granular material was pushed into the ground or the ground surface 7
has shown significant swelling which means that subsequent tamping would only push 8
granular material sideways rather than downwards. From site observations, columns built 9
using DR method were limited to between 4 and 6m. 10
11
Some aspects of construction that should be taken into account in the design are 12
described herein. Firstly, installation of the column is by extreme displacement of the 13
insitu soils, within an instantaneous time frame. Hence the surrounding soil will be 14
displaced sideway, upwards (heave) and downwards which means that there is no 15
improvement to the surrounding soil. The soft soils displaced downwards imply that there 16
is a layer of soft soils (about 1 to 1.5 m thick) below the toe of the column. This layer 17
needs to be consolidated by surcharging. Figure 5a shows the soft layer formed below the 18
toe of DR columns as detected by the post treatment CPT. 19
20
Figure 4. Schematic granular column formed by dynamic replacement method 21
1
2
3
4
5
6
7
8
5a. Pre and Post treatment CPT indicating 5b. Post treatment CPT from centre of 9
remoulded zone below DR column DR column to prove 2.5 m dia. column 10
7.0 Quality Assurance and Quality Control 11
7.1 Prefabricated Vertical Drain (PVD) 12
The QA/QC plan implemented on site comprised of 3 stages, namely pre-treatment, 13
during execution and post-treatment. Pre-treatment testing (e.g. CPT and DPT) were 14
carried out to establish the depths of soft soil. During the execution stage, the depth of 15
installation was monitored independently and cross checked with material delivered to 16
site on a daily basis to ensure sufficient depth as required by design was achieved. 17
Finally, the embankment was monitored during construction to ensure that consolidation 18
was occurring as per design requirement. 19
7.2 Vibro Sand Columns (VS) 20
The QA/QC plan for Vibro Sand Columns covers the construction methodology, material 21
use, termination criteria and compaction effort to ensure desired diameter were achieved. 22
Appropriate vibrator with sufficient centrifugal force (about 15 tons) was used. Material 23
delivered to site was tested for specified grading at every 2,000 tons interval. The volume 24
of bucket used to transfer sand into the ground was calibrated. The construction process 25
was recorded in real time basis by a computer to ensure sufficient compaction of sand to 26
build desired column diameter. Pre-treatment S.I. was used to calibrate the responding 27
power consumption by the vibrator when a medium stiff to stiff layer was encountered 28
for the first column installed. Subsequent installation would then be terminated at a depth 29
where the agreed power usage was achieved. During compaction of column, each meter 30
of column built was checked to achieve the required power usage in real time. At the 31
completion of installation, CPTs were carried out at the center of column to ensure 32
continuity and columns of adequate density (qc>5MPa). Staged construction was adopted 33
for areas treated with VS. The embankment construction was monitored to ensure 34
Remoulded
Zone
Post-treatment
Remoulded
Zone
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0
Tip Resistance Qc [MPa]
Depth [M]
Pre-treatment
Post-treatment CPT
0m, 0.5m & 1m from
the centre of column
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0
Tip Resistance Qc [MPa]
Depth [M]
Post-treatment CPT
1.5m from the centre
of column
stability and consolidation was occurring as per design requirement. Figure 6 shows the 1
settlement versus time plot of a typical embankment on VS. In general, the 2
compressibility of the VS treated soil is halved compared with soil treated with PVD. 3
Figure 6. Typical settlement versus time plot on VS at East Coast Expressway 4
7.3 Vibro Stone Columns (VR) 5
The QA/QC plan implemented for VR was similar as per VS. Stone material used for VR 6
was tested for specified grading and strengths. The embankment was constructed at 7
specified rate of filling and was monitored during the construction to ensure the stability 8
and consolidation was occurring as per design. Generally the compressibility of the 9
treated area was half of that of the area treated using vertical drains, besides a much 10
shorter consolidation time. 11
7.4 Dynamic Replacement Columns (DR) 12
A stringent quality plan was implemented since this method is less frequently employed 13
for soft ground treatment. Firstly, pre-treatment soil investigations (CPT and PMT) were 14
carried out to establish that the depths of soft soil were less than 6m. Where the soils 15
were deeper than 6m, alternative treatment methods were specified by the consultant (e.g. 16
stone column). Secondly, site trials were carried out to ascertain that 2.5m diameter could 17
be formed to 6m depth (see Figure 5b). 18
19
During execution, the tamper drop height and number of blows were measured by an on 20
board computer. Pre-programmed computer sequencing allowed consistent repeated 21
drops without reliance on the operator’s reflexes to adjust lift heights and apply brakes. 22
This enhances not only the quality of the product but safety on site. 23
-500
-450
-400
-350
-300
-250
-200
-150
-100
-50
0
0 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375 400
Days
Settlement (m
m)
Monitoring Works for East Coast Expressway - LPT2
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
Embankment Heights (m)
1
Post-treatment CPT and PMT tests were carried out to ascertain column quality. The CPT 2
was found to be more effective QA/QC tool than the PMT as it measures soil 3
characteristics (strength and pore water pressure) continuously. The results are also found 4
to be more repeatable and easier to interpret. 5
6
The embankment was also monitored during construction to ensure that consolidation 7
was occurring as per design requirement. 8
8.0 Conclusion 9
Ground improvement techniques applied as foundation for highway projects are widely 10
accepted and are increasing in application in Malaysia. This paper presents the 11
application of ground improvement techniques (i) Prefabricated Vertical Drain (ii) Vibro 12
Sand Column (iii) Vibro Stone Column and (iiii) Dynamic Replacement in ECE 2. The 13
concepts behind each technique are discussed, as well as their respective design 14
methodology, execution, and QA/QC plan. Generally, ground improvement techniques 15
were found to be adequate in supporting high embankments without instability and 16
settlement issues. The selection of suitable method is dependant on factors such as 17
embankment height, soil condition (strength and depth), availability of material and costs. 18
Acknowledgments 19
The authors wish to thank the Public Work Department and Malaysia Highway 20
Authorities for allowing us to participate in this important project. We would also like to 21
acknowledge the management and staff of the Main Contractors and Consultants (MTD 22
Capital Berhad, GPQ-Bukit Puteri JV Sdn Bhd, Tidal Marine Sdn Bhd, TSR Bina Sdn 23
Bhd, Cergas Murni Sdn Bhd, HSSI Integrated Sdn Bhd, Minconsult Sdn Bhd, Terratech 24
Consultants Sdn Bhd and WNA Consultant Sdn Bhd) for their valuable contribution in 25
the implementation of the ground improvement works. Colleagues in Keller who 26
contributed immensely in the design and construction, in particular: Dr. V.R. Raju, Mr. 27
Saw Hong Seik and Mr. Sreenivas are also appreciated. 28
References 29
Priebe, H.J., The Design of Vibro Replacement, Ground Engineering: p.31–37, (1995) 30
31
British Standard: BS EN 14731:2005, Execution of special geotechnical works – Ground treatment 32
by deep vibration., 2005. 33
34
Building Research Establishment, Specifying dynamic compaction. BRE458. Garston, BRE 35
Bookshop, 2003. 36
37
Yee, Y.W. and Raju, V.R., Ground Improvement Using Vibro Replacement (Vibro Stone Columns) 38
– Historical Development, Advancements and Case Histories in Malaysia, 16th Southeast Asian 39
Geotechnical Conference, Kuala Lumpur, Malaysia, 2007. 40